Solar power towers are the new trend in thermal power plants. Instead of rows of curved mirrors focusing sunlight onto miles of black tubing, power towers use Heliostat (sun-tracking) mirrors to focus light on a central tower. As first conceived, that “power tower” would contain water that when heated, creates steam to spin a turbine and generate electricity. Two new solar power towers set to be built by SolarReserve will make one major improvement: instead of using water in the tower, they use molten salts.
The advantage of molten salt is its high heat capacity, allowing it to get much hotter during the day and retain much of that heat throughout the night. The result is a clever form of solar energy storage that could help transform solar into a base-load, on-demand power source. SolarReserve is introducing the concept to the desert Southwest. A 100-megawatt plant will be built in Nevada under a power purchase agreement (PPA) with NV Energy. A second, 150-MW power tower will come as part of a deal with Pacific Gas & Electric (PG&E).
The Nevada plant, dubbed the Crescent Dunes Solar Energy Project, will have at its center a 538-foot-tall concrete tower topped with a collector containing millions of gallons of molten salt. That liquefied salt, an eco-friendly mixture of sodium and potassium nitrate, is heated to more than 1,000 degrees during the day. Crescent Dunes will have up to 12 hours of storage capacity, allowing near round-the-clock draw. The California project will have seven hours.
Power towers are still in their early stages. Some already exist around the world, including a few molten salt towers in use in Spain, but those are at best 20 MW in size. The ability to successfully store solar electricity created by these power plants could effectively double the usefulness of solar thermal power. According the National Renewable Energy Laboratory, electricity from power towers could sell for 5.47 cents per kilowatt-hour by 2020.
Many solar thermal plants in US deserts are mired in controversy due to environmental fears surrounding damages to landscape and habitat. Compared to conventional solar thermal power plants, power towers demand a smaller footprint. Their ability to store electricity allows them to create even more electricity with less space. Scarce water supplies may be an issue. Some power towers are air-cooled rather than water-cooled to preserve precious local resources.
Photo & Source: New York Times